a) Field of the Invention
The present invention relates to a semiconductor laser, and more particularly to a semiconductor laser having an excellent conversion efficiency.
b) Description of the Related Art
Optical fiber communications have spread from large capacity trunk systems to subscriber systems. Low cost systems are essential for subscriber optical fiber communications. As a light source, a Peltier-free system which requires no cooler is desired.
From this viewpoint, a semiconductor laser has long been desired which can emit a sufficiently large light output at a low drive current even at a high temperature of near 85.degree. C. It is therefore necessary to raise a characteristic temperature of a semiconductor laser sufficiently high because the characteristic temperature governs the temperature characteristics of the semiconductor laser.
Semiconductor lasers have been fabricated heretofore on a GaAs substrate or an InP substrate. Semiconductor lasers for optical fiber communications are required to have an emitted light wavelength of a 1.3 .mu.m band or 1.55 .mu.m band (collectively called 1 .mu.m band) at which an optical fiber has less loss.
Light emission of the 1 .mu.m band cannot be obtained by a semiconductor laser which uses an epitaxial layer lattice-matching and formed on a GaAs substrate. InP substrates have been used therefore for semiconductor lasers of the 1 .mu.m band.
A typical semiconductor laser of the 1 .mu.m band uses InGaAsP/InP based materials utilizing an InP substrate and InGaAsP active layer. It is desired to form a clad layer having a wide band gap in order to confine carries in and near an active layer. InP has been used because this material has the widest band gap among group III-V compound semiconductors which lattice-match an InP substrate.
A guide layer (light waveguide layer, barrier layer) For confining light in and near the active layer is interposed between the clad layer and active layer. As the guide layer, InGaAsP having a wider band gap than the active layer has been used.
However, a characteristic temperature of such an InGaAsP/InP semiconductor laser is about 60 K and steeply lowers its light output at a high temperature.
Depending on materials having relatively narrow and wide band gaps used to form a potential barrier between the active layer and barrier layer, a ratio (.DELTA.Ec/.DELTA.Ev) of a band discontinuity (.DELTA.Ec) between conduction bands to a band discontinuity (.DELTA.Ev) between valence bands changes.
The effective mass of an electron is lighter than a hole. It is therefore desired that .DELTA.Ec is larger than .DELTA.Ev so as to efficiently confine electrons arid holes in the active layer. However, if lattice-matched layers having different band gaps are formed by InGaAsP/InP based materials, .DELTA.Ev becomes considerably larger than .DELTA.Ec.
Semiconductor lasers with an improved performance at a high temperature have been developed by forming a clad layer and guide layer with AlInGaAs based materials. By using AlInGaAs based materials, .DELTA.Ec can be made large and the temperature characteristics can be improved to some degree.
However, the characteristic temperature obtained by using AlInGaAs based materials is not still satisfactory. In order to make the characteristic temperature sufficiently high, it is desired to make the value of .DELTA.Ec larger than the value of .DELTA.Ev by the degree corresponding to the difference between the hole effective mass and hole effective mass.
It is known that a lasting threshold current of a semiconductor laser lowers if an active layer with strains is used. There is also known a strain MQW (multi quantum well) semiconductor laser using a strain active layer of a superlattice structure. The temperature dependency of a strain MQW semiconductor laser has not been improved as yet.
It has been proposed to make opposite reflection surfaces of a cavity of a semiconductor laser have a high reflectance so as to realize a low oscillation threshold value even at a high temperature. Since the intensity of light reciprocally moving in the cavity increases, a drive current can be reduced. However, a high reflectance cavity lowers a light output.
With conventional techniques described above, it has been difficult to realize semiconductor lasers of the 1 .mu.m band having a high characteristic temperature.